Computer guided surgery (CGS) is a tool often used by surgeons to assist during surgical procedures. When using CGS, the bone structure and/or the internal anatomy of a patient is known to a navigation system following a reading-in procedure. CGS, via a display, provides visual feedback to the surgeon of an area of interest in the patient with respect instruments, such as awls, chisels, bodkins and/or other devices (e.g., stability rods, tools, clamps and the like) as well as the position of the instruments in the area of interest. The surgeon uses the visual feedback to place or otherwise position the instruments on or in the area of interest to achieve a desired result and/or position of the instrument with respect to the area of interest.
In CGS, instruments are calibrated intra-operatively on or in the area of interest of the patient. Once calibrated, the instrument can be navigated to achieve the desired position of the instrument with respect to the area of interest. To this end, an instrument referencing device, such as a reference star, is clamped or otherwise attached to the corresponding instrument, and the reference star, in conjunction with a particular calibration block (e.g., an instrument calibration matrix (ICM)—if necessary), is used to calibrate the instrument within a work space. All in all, this calibration step can be very time-consuming and inconvenient for the user. Moreover, there exists a possibility that the referencing device can come loose after calibration, i.e., during application of the instrument, thereby distorting the calibration and thus the accuracy of navigation. In addition, the calibration itself can conceal possible sources of error that surface later, resulting in further navigation inaccuracies. If the user is not aware of these inaccuracies in the instrument calibration, injuries to the patient are possible.
In CGS on the spine, navigation is mainly used to accurately place implant screws into the pediculi of the vertebra, without injuring the spinal cord or spinal nerves or other vital organs or blood vessels in the surrounding area. The spinal instruments serve to prepare and open up the pediculus in order to correctly insert the pedical screws. Pre-calibrated instruments generally are required for CGS. By pre-calibrating the instruments, the chance for calibration errors introduced by the user are minimized or eliminated altogether.
A rotatable instrument adaptor for spinal CGS is known from U.S. Pat. No. 6,021,343 to Foley et al. This solution includes an intermediate piece adaptor having a rotatably mounted reference star that is recognised by the navigation system. The adaptor includes a quick-release lock at each of its two ends. On one end, different instrument tips can be attached to the adapter, and on the other end different grips or drills can likewise be attached. The intermediate piece adaptor then can be used to navigate different spinal instruments, which, however, have to be rotationally symmetrical in order to be correctly displayed.
U.S. Pat. No. 6,556,857 to Estes et al. mentions how a rotatable intermediate piece adaptor can have a catch mechanism in a position known by software. However, the embodiment and functionality of such a catch mechanism for navigation are not described.